In the field of current semiconductor elements, there is a growing tendency of lessening the weight, decreasing the thickness, reducing the size of elements and manufacturing such elements in a small lot-high variety manner. As a result of this tendency, the lead frame used for mounting IC chips on substrate is required to have an increased number of pins in a more slender and smaller size. On the other hand, the stamping process is unable to produce such small-sized articles satisfying the requirement of the miniaturization because of elevated cost of metallic die needed by the small lot-high variety production. Thus, the etching process is advantageous over the stamping process for the small lot-high variety production because it can produce small-sized articles without using any metallic die. The etching process uses a photosensitive resin in either of the three forms, namely water-soluble photosensitive resin, solvent-containing liquid photosensitive resin and photosensitive film.
The use of liquid photosensitive resin has general faults in that an enormous investment must be made in the coating apparatus, the coating process requires considerably much labor, serviceable lives of the photosensitive resin itself and the coated film of photosensitive resin are short, and sensitivity is low. In addition, individual cases have their own faults. For instance, the water-soluble liquid photosensitive resin in which casein or PVA (polyvinyl alcohol) is cured with a chromic acid salt has a problem that a complicated step of waste water treatment must be provided for the disposal of harmful heavy metal salt after use. On the other hand, the solvent-containing liquid photosensitive resin has a problem that an organic solvent is discharged into the atmospheric air in the course of coating, which exercises an adverse influence upon environments.
On the other hand, the photosensitive film has a sandwich structure in which a photosensitive resin composition is coated on a transparent support film, dried and then covered with a protecting film. At the time of lamination, the protecting film is removed and at the same time the photosensitive resin layer is thermally press-bonded to the underlying metal to form an image. Accordingly, the photosensitive film is lower in the cost for equipments, higher in sensitivity and longer in the working life than the above-mentioned liquid photosensitive resins, and is superior in the suitability for metallic precision fabrications.
As the support film of the photosensitive film, polyester films such as PET (polyethylene terephthalate) film are generally used. As the protecting film, polyolefin films such as PE (polyethylene) film are generally used. The protecting film is removed at the time of lamination. The polyolefin film conventionally used as a protecting film is produced by thermally melting and kneading a raw material and then forming it by extrusion, biaxial orientation or casting. In general, protecting films of polyolefin or the like include unmelted materials or thermally deteriorated regions called fish eyes. The fish eye usually has a diameter (φ) of 30 to 600 μm, forming a protrusion on the film surface up to a height of 2 to 40 μm. The convex portions of the fish eyes are transferred onto the photosensitive resin layer to form concavities on the photosensitive resin layer, so that air voids 6 are formed on the substrate after lamination as seen in FIG. 1B. That is to say, if a photosensitive film comprising a support film 1, a photosensitive resin layer 2 and a protecting film 3 carrying fish eyes 4 (see: FIG. 1A) is laminated, after releasing the protecting film 3 therefrom, onto a substrate 5, air voids appear as seen in FIG. 1B. Formation of the air voids has a relation with film thickness of the photosensitive resin layer so that a smaller film thickness of photosensitive resin layer causes a more ready formation of air voids. The presence of such air voids causes formation of defective pattern and breakage of wire in the subsequent steps of exposure, development and etching.
As a means for preventing such phenomena, JP-A 3-12402 discloses the use of a film having a flat and smooth surface as the releasable film at the time of lamination. The method of JP-A 3-12402, however, is characterized by coating a photosensitive resin composition onto a releasable film to be peeled off at the time of lamination and drying the coat to form a photosensitive resin layer, followed by laminating thereon a support film. The releasable film must be selected from materials showing no thermal dimensional change when it is coated with a photosensitive resin composition and dried. In other words, the materials usable for this purpose are restricted.
Apart from the above, the method of vacuum lamination is useful as mentioned in JP-A 52-66581, JP-A 51-63702, and JP-A 1-314144. This method, however, is disadvantageous in that it needs an apparatus of larger size as compared with the conventional normal pressure lamination method and it tends to generate dusts because inner atmosphere of lamination chamber is kept at a vacuum.